A fuel cell is an electrochemical cell that converts a source fuel into an electrical current and water. It generates electricity inside a cell through reactions between a fuel and an oxidant, triggered in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
A fuel cell comprises a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte interposed between the electrodes and means for separately supplying a stream of fuel and a stream of oxidizer to the anode and the cathode, respectively. In operation, fuel supplied to the anode is oxidized releasing electrons which are conducted via an external circuit to the cathode. At the cathode the supplied electrons are consumed when the oxidizer is reduced. Proton exchange membrane fuel cells use a solid proton-conducting polymer membrane as the electrolyte.
Applications for fuel cells include battery replacement; charging of mini- and microelectronics such as portable electronic devices; sensors such as gas detectors, seismic sensors, and infrared sensors; electromechanical devices; automotive engines and other transportation power generators; power plants, and many others.
Electrochemical fuel cells can employ a variety of gaseous fuels and oxidants. For example, hydrogen fuel cells employ molecular hydrogen as the fuel and oxygen in air or a carrier gas as the oxidant. In hydrogen fuel cells, hydrogen gas is oxidised and oxygen gas reduced to form water, with an electrical current produced as a by-product of the redox reaction. Electron flow along the electrical connection between the anode and the cathode provides electrical power to load(s) interposed in the circuit with the electrical connection between the anode and the cathode. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:Anode reaction: H2→2H++2e−Cathode reaction: ½O2+2H++2e−→H2O
Hydrogen fuel cells usually employ a catalyst at the electrodes, especially the anode, to increase the rate of the redox reaction. Such catalysts typically comprise noble metals such as ruthenium, rhodium, palladium, silver, osmium, iridium, platinum or gold. However, these metals are scarce and therefore expensive.
In particular, palladium is commonly used in catalysts for redox reactions involving hydrogen. However, palladium is known to absorb electrolytically produced hydrogen to form a hydride: the hydride forms a rapidly disintegrating powder which makes it unsuitable for catalysis.
It would therefore be desirable to produce a material suitable for catalysing the redox reactions taking place in a hydrogen fuel but is cheaper than the materials currently used.
Greeley et al., Nature Materials, 2006, 5, 909-913, describes a computational high-throughput screening method of electrocatalytic materials for hydrogen evolution, and describes in detail a platinum/bismuth alloy and its catalytic activity in hydrogen evolution.
WO 2008/080227 describes a number of different catalysts, especially platinum/tin and palladium/bismuth catalysts, and their use in a number of electrochemical oxidation of organic compounds, in particular formic acid.
U.S. Pat. No. 4,843,173 describes palladium-bismuth/carbon catalysts and their use in oxidising glucose to gluconic acid.
BE 883331 describes palladium-bismuth intermetallic compounds, including PdBi2. The compounds are stated to be useful as catalysts for converting aldehydes and alcohols into carboxylic esters.
U.S. 2007/0167318 describes catalytically active compositions comprising Pd, Bi and at least one other element. The compositions are stated to be useful in dehydrogenating hydrocarbons.
WO 2004/012290 describes intermetallic compounds for use as catalysts and catalyst systems. The intermetallic compounds are stated to be useful for catalysing reactions in fuel cells.
GB 1559700 describes electrochemical cells capable of catalysing the conversion of a hydrocarbon containing at least one hydroxyl group, in which at least one electrode comprises a composition comprising bismuth and one or both of platinum and palladium.